Support for reftables in git repos

[vraravam]

Git (3.0) will be converting to use reftable format.

I tried for an existing git repo (copied to a temp folder), and the prompt shown is unable to determine the branch name

(the first prompt with invalid is for the same repo converted to using reftables, whereas the 2nd prompt shows for the non-reftable clone)

[mdcclv]

Seems to be dependent on this change in libgit2: libgit2/libgit2#7117

[NN---]

I discovered that all my repositories are with the new format.
It took some time to understand why powerlevel does not display branches.

[Freed-Wu]

Before it is fixed, you can:

git refs migrate --ref-format=files